The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 199 62 706.1, filed on Dec. 23, 1999, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a press section of a machine for producing tissue webs, having a shoe press containing a shoe press unit and a counter roll, whose shoe press unit has at least two press shoes and an elastic liquid-impermeable pressing belt guided over these press shoes.
2. Discussion of Background Information
From DE 32 05 664 A1, a shoe roll with at least two press shoes is known for use in a press section of a paper machine, for example. The press shoes are pivotable around an axis parallel to the pressing plan and can differ in their bending radius. This allows optimal press performance to be achieved for the entire thickness spectrum of the sorts of paper produced on a paper machine, i.e., as high a dry content as possible can be achieved.
However, since tissue webs are very thin, the transmission of this knowledge does not result in an improvement of the product quality in the tissue production. Between the different types, only irrelevant thickness differences occur. The application is therefore uneconomical due to the larger investment expense of the described shoe press.
The present invention provides a press section of the above-mentioned type that allows an economical production of tissue and simultaneously ensures a better product quality.
According to the invention, the two press shoes have different lengths measured in the web travel direction and can each be transferred from an idle position to a press position in which the press shoe in question is positioned opposite the counter roll.
With the press shoe being transferable between an idle position and a press position, press nips of different lengths can be realized quickly and without any reconfiguration expense.
Depending on use, different quality standards are required from tissue products. There are products for which the specific volume (xe2x80x9cbulkxe2x80x9d) is a deciding quality feature. In other products, it can be more important for economical reasons to accept a lower specific volume in exchange for a high dry content, i.e., a high production. Because of the embodiment according to the invention, it is now possible to change between these types in the tissue production without having to compromise concerning the adjustment of the machine setting and the press setting, as previously necessary. Now the optimal situation can be achieved in each case.
A shoe press roll with a press jacket forming the press belt is provided as the shoe press unit.
For achieving a high specific volume, it is advantageous if at least one press shoe is provided whose length measured in the web travel direction is greater than about 80 mm, greater than or equal to about 100 mm, preferably greater than or equal to about 120 mm. Here, the highest pressure of the pressure progression occurring in a press nip between the press shoe and the counter roll in the web travel direction is preferably selected to be less than or equal to a value of about 2.5 MPa, in particular less than or equal to about 2 MPa, preferably less than or equal to about 1.5 MPa.
On the other hand, it is advantageous for achieving as high a dry content as possible after the press when at least one press shoe is provided whose length measured in the web travel direction is less than or equal about 60 mm and, in particular, less than or equal to about 50 mm and preferably has a value in the range of about 37 mm. Here, the maximal pressure of a pressure progression occurring in a press nip between the press shoe and the counter roll in the web travel direction in a shoe length less than or equal to about 60 mm is preferably selected to be greater than or equal to a value of about 3.3 MPa, at a shoe length less than or equal to about 50 mm preferably greater than or equal to a value of about 4.3 MPa and, at a shoe length in the range of about 37 mm, preferably equal to a value in the range of about 4.8 MPa.
It is particularly advantageous if a heated drying cylinder is provided as a counter roll, particularly a so-called Yankee cylinder that has a considerably larger diameter than the shoe press roll if the shoe press unit is formed by a shoe press roll.
When using the Yankee cylinders, the maximal linear force is frequently limited to a value of about 90 kN/m to 120 kN/m due to excessively large deformations and tensions that occur in a Yankee cylinder. This again limits the maximal pressure for a press shoe designed for high bulk and thus the maximum achievable dry content for the tissue web. This results in a reduction of the machine speed and a higher drying expense at the drying cylinder. By using a shoe press with at least two different press shoes according to the invention, this disadvantage is omitted.
A further increase in dry content can be achieved in that, in a press nip surfaced with felt formed between a shoe press unit and a counter roll, preferably a drying roll, the angle between the tangent coming in contact with the counter roll at the end of the press nip and at least one of the felts leaving the press nip is advantageously greater than or equal to about 10xc2x0, in particular greater than or equal to about 15xc2x0, in particular greater than or equal to about 18xc2x0, and preferably greater than or equal to about 20xc2x0. In such a case, particularly, the press belt can be provided with a grooved (ribbed) surface in the circumferential direction that additionally aids in the increase of the dry content. A ribbed press belt is therefore preferably used when a in tissue machine tissues are produced that mainly require a high cry content.
Otherwise, a blind-bored and/or grooved press belt can be used in particular.
The present invention is directed to a press section of a machine for producing webs. The press section includes a shoe press including a shoe press unit and a counter roll. The shoe press unit includes at least two press shoes and an elastic liquid-impermeable press belt guided over the at least two press shoes, and the at least two press shoes have different lengths measured in a web travel direction and each of the two press shoes is arranged for movement between an idle position and a press position facing the counter roll.
In accordance with a feature of the instant invention, the web can include a tissue web.
According to another feature of the invention, the shoe press unit may include a shoe press roll having a press jacket arranged to form the press belt, and the shoe press roll can include the at least two press shoes.
At least one of the at least two press shoes may have a length in the web travel direction greater than about 80 mm. A maximal pressure of a pressure progression occurring within a press nip formed between the at least one press shoe and the counter roll can be less than or equal to a value of about 2.5 MPa.
At least one of the at least two press shoes may have a length in the web travel direction less than or equal to about 60 mm. A maximal pressure of a pressure progression occurring within a press nip formed between the at least one press shoe and the counter roll can be greater than or equal to a value of about 3.3 MPa.
At least one of the at least two press shoes may have a length in the web travel direction less than or equal to about 50 mm. A maximal pressure of a pressure progression occurring within a press nip formed between the at least one press shoe and the counter roll can be greater than or equal to a value of about 4.3 MPa.
At least one of the at least two press shoes has a length in the web travel direction less than or equal to about 37 mm. A maximal pressure of a pressure progression occurring within a press nip formed between the at least one press shoe and the counter roll can be greater than or equal to a value of about 4.8 MPa.
In accordance with a further feature of the present invention, the counter roll may include a heatable drying cylinder. Further, the heatable drying cylinder can include a Yankee cylinder.
Further, a press felt can be guided through a press nip formed between the shoe press unit and the counter roll. The press felt may be guided from an end of the press nip at an angle of greater than or equal to about 10xc2x0 to a tangent to the counter roll at the press nip exit. Moreover, the press felt can be guided from the end of the press nip at an angle of greater than or equal to about 15xc2x0 to the tangent, the press felt can be guided from the end of the press nip at an angle of greater than or equal to about 18xc2x0 to the tangent, and preferably, the press felt may be guided from the end of the press nip at an angle of greater than or equal to about 20xc2x0 to the tangent. The press belt can have a grooved surface in a circumferential direction.
In accordance with still another feature of the invention, the press belt can be one of blind bored and grooved.
According to another feature of the invention, a thin fabric may be arranged between the at least one press felt and the web. The thin fabric can be one of a dewatering wire and an imprinting wire. Further, the thin fabric may have a thickness less than about 2 mm, and advantageously less than about 1 mm.
According to a still further feature of the instant invention, the at least two press shoes may be arranged for alternative positioning in the press position.
Moreover, the shoe press unit can include a rotatable carrier, and the at least two press shoes are coupled to the rotatable carrier. The rotatable carrier may be arranged to alternatively move the at least two press shoes between the idle position and the press position.
The instant invention is directed to an apparatus for producing webs. The apparatus includes a shoe press unit including at least two press shoes having different lengths in a web run direction and a flexible, liquid impermeable press belt guided over the at least two press shoes, and a counter roll. The at least two press shoes may be alternatively positionable to form a press nip against the counter roll.
In accordance with a feature of the present invention, a first of the at least two press shoes can have a length in the web travel direction greater than about 80 mm and a second of the at least two press shoes can have a length in web travel direction of less than or equal to about 60 mm. A maximal pressure of a pressure progression occurring within the press nip formed between the first press shoe and the counter roll may be less than or equal to a value of about 2.5 MPa, and a maximal pressure of a pressure progression occurring within the press nip formed between the second press shoe and the counter roll may be greater than or equal to a value of about 3.3 MPa.
Moreover, the second press shoe may have a length in the web travel direction less than or equal to about 50 mm, and the maximal pressure of a pressure progression occurring within the press nip formed between the second press shoe and the counter roll can be greater than or equal to a value of about 4.3 MPa.
Further, the second press shoe can have a length in the web travel direction less than or equal to about 37 mm, and the maximal pressure of a pressure progression occurring within the press nip formed between the second press shoe and the counter roll may be greater than or equal to a value of about 4.8 MPa.
According to still another feature of the invention, the shoe press unit may include a rotatable carrier, and the at least two press shoes can be coupled to the rotatable carrier.
In accordance with a further feature of the present invention, the rotatable carrier may be arranged to alternatively move the at least two press shoes between the idle position and the press position.
The present invention is directed to a process for producing a web in an apparatus that includes a shoe press unit and an counter roll, the shoe press unit includes at least two press shoes having different lengths and an elastic liquid-impermeable press belt guided over the at least two press shoes. The process includes alternatively positioning one of the at least two press shoes into a press position to face the counter roll, whereby the other of the at least two press shoes is positioned in an idle position, and guiding the web through a press nip formed between the one press shoe in the press position and the counter roll.
In accordance with a feature of the instant invention, a first of the at least two press shoes may have a length in the web travel direction greater than about 80 mm and a second of the at least two press shoes may have a length in web travel direction of less than or equal to 60 mm, and the process can further include alternatively applying a maximal pressure of a pressure progression within a press nip formed between the first press shoe and the counter roll less than or equal to a value of about 2.5 MPa, and applying a maximal pressure of a pressure progression within a press nip formed between the second press shoe and the counter roll greater than or equal to a value of about 3.3 MPa.
Further, the second press shoe can have a length in the web travel direction less than or equal to about 50 mm, and the process can further include applying a maximal pressure of a pressure progression within the press nip formed between the second press shoe and the counter roll greater than or equal to a value of about 4.3 MPa.
Moreover, the second press shoe can have a length in the web travel direction less than or equal to about 37 mm, and the process may further include applying a maximal pressure of a pressure progression within the press nip formed between the second press shoe and the counter roll greater than or equal to a value of about 4.8 MPa.
In accordance with yet another feature of the instant invention, the apparatus may further include a press felt guided through the press nip, and the process may further include guiding the press felt from an exit of the press nip an angle greater than or equal to about 10xc2x0 to a tangent to the opposing roll at the exit of the press nip.
In accordance with still yet another feature of the present invention, the apparatus further includes a thin fabric arranged between the web and the felt, and the process can further include deflecting the thin fabric away from the web at an angle less than or equal to the angle of the felt.